1. Field of the Invention
This invention is directed to a method and device for measuring the performance of an optical amplifier and more precisely to a method of determining optical amplifier failures.
2. Background Art
Modern optical transmission systems comprise a transmitting and a receiving terminal connected through a strand of optical fiber. The optical signal is attenuated as it travels along the optical fiber, which limits the possible spacing of the terminals. To achieve long haul transmission at high line rates, regenerators (repeaters) and/or optical amplifiers are deployed along the optical transmission link in multiple locations, for boosting the signal on the fiber. For systems operating at data rates of a few GBps, regenerator sites could be spaced in the range between 35 to 80 Km, depending on the wavelength chosen for transmission. Typically, in the line amplifying configuration, the distance between the optical amplifiers may be almost doubled, being in the range between 80 to 160 Km. This distance is determined by the optical power launched into the fiber by the upstream amplifier, the loss and dispersion of the optical fiber interconnecting this amplifier with the closest downstream amplifier, and the sensitivity of the downstream amplifier.
The use of fiber amplifiers in long-distance systems is favored not only by the longer distances between the modules, but also because optical amplifiers do not require conversion of the optical signals traveling along the fiber into electrical signals and back, and also because they are spliced easily into the fiber transmission link. While the performance of opto-electronic regenerators may be measured by monitoring the characteristics of the regenerated electrical signal, measuring the performance of the optic amplifiers requires specific monitoring techniques, because regenerated electrical signals are not available.
In an optical amplifier there are some faults that degrade performance by increasing the optical noise rather than by decreasing the net gain. The increased optical noise causes bit errors at the terminals of the transmission system even though the received optical signal strength meets design objectives. Moreover, failures in the optical components of an amplifier may degrade the optical performance while not preventing the amplifier from obtaining the desired output power. It is valuable to detect these degradations as they can cause excessive noise, reduce margin in performance features, or be an advance warning of further failures.
The failures at an Erbium Doped Fiber Amplifier (EDFA) may be classified as electronics failures, pump laser failures and optical component failures. For example, variations of pump laser wavelength due to the aging or to malfunctions of the temperature control system can increase the optical noise. Excessive losses or total loss of the input signal need to be clearly detected, because such losses are generally compensated by higher gain toward the output port of the EDFA, so that the net effect is a decreased signal to noise ratio of the output signal.
The amount of gain that a given optical amplifier may generate is not only a function of the input power, but also a function of the output power due to saturation of the gain at higher powers. Currently, the output power levels in an optical amplifier can be provisioned by the user for allowing flexibility in the design configuration of the optical transmission system. Miscalculations in the design of transmission system configuration may occur in that, for example, a higher output power than the amplifier can give may be provisioned by the user.
When an optical amplifier is unable to obtain the desired output power it may be because the amplifier has degraded, because the input signal is too small for the desired output power, or it may be because the provisioned power set by the user is higher than the design parameter. It is valuable for the user of the system to be able to accurately distinguish these cases, should the desired output power not be obtained at any of the amplifiers in the transmission link.
When an amplifier has more than one output direction or band, and has the ability to control the power separately per band, then another problem can occur. The amplifier may run out of control dynamic range for a given set of input signal power levels, and not be able to simultaneously keep all of the output power levels at their provisioned values. One level may be too high and another too low, for example. This dynamic range is, in general, a function of the input power levels and the provisioned output power levels. It is valuable for the system manager to distinguish between a failure of an amplifier and a set of input and provisioned output conditions that the amplifier is not designed to satisfy.
Some faults are currently signaled. Thus, generating a loss of signal (LOS) alarm when the input signal has less power than a given threshold is standard practice in fiber-optic transmission equipment. As well, generating alarms when an amplifier is unable to meet the provisioned gain or, alternatively, the provisioned output power, is also known.
U.S. Pat. No. 5,513,029 (Roberts et al., issued Apr. 30, 1996 and assigned to Northern Telecom Ltd.) discloses a method of measuring the optical signal to noise ratio generated by an amplifier if the appropriate dither is present on the laser source.
However, the prior art generally fails to provide a method for detecting degradations and faults in the optical components of an amplifier, and distinguishing between equipment failures and lack of an input signal. The present invention is concerned with such methods and devices.